The addition of solid additives to a polymer material is typically accomplished by dry blending or dry mixing or by direct incorporation, particularly solution or melt blending. The method chosen depends upon a number of factors including the relative size of the additive particle to the polymer particles, the sensitivity of the additive material to the process steps by which the addition is to be attained as well as to the materials used, and the desired property or objective of the additive itself. Each method has its attributes and its problems or issues: none is ideal for all circumstances.
Dry blending is perhaps the most simplest and cost effective means of producing a binary system of two solid particle materials. The polymer particles and the solid additive particles are merely placed in a vessel and the vessel rotated, like a cement mixer, to intimately mix the two and/or a mixer means is incorporated or inserted into the vessel to mix the components. This process is especially suited for mixing polymer particles and additive particles of approximately the same size and mass. Because of their similar size, once the distribution is established, the distribution will remain fairly stable, regardless of shaking or vibrations. On the other hand, when the additive particles are of disparate size, especially where the sizes are orders of magnitude in difference, it may be impossible to achieve a truly stable, homogeneous distribution as the smaller particles will continually cascade down through and settle from the larger particles during the mixing process. Thereafter, the cascading will tend to continue as the mixture is moved, shaken or otherwise subjected to vibrations and the like.
Nonetheless, where the particle size difference is marked and the quantities of the additive are not overly excessive, one may achieve a fairly uniform coating of the polymer particles with the additive particles, much like the addition of powdered cinnamon to granular sugar. However, the adherence of the one to the other is typically reliant upon Van Der Waals forces and/or electrostatic charges, both of which are relatively low in strength. In the absence thereof, and even in the presence thereof due to the inherent weakness of such forces and charges, dissociation and settling occurs, particularly during handling and transport of thereof. Thus, without proper remixing of the composition prior to use, portions thereof will have little or no additive while others will have excessively high levels of the additive. For example, in molding applications, articles made with materials taken from the top of the storage container will have higher loadings of the additive than those made from materials taken from the bottom of the storage container.
Depending upon the intended processing and/or end-use of these materials, even with proper remixing, the additive materials may readily dissociate from the surface of the polymer particles due to shock, vibration or shaking in subsequent processing steps, application steps or use. Similarly, if the materials are to be used in a flow environment, the force of the flow stream, whether a gas, such as air; a liquid such as water, or even a flowable solid, will tend to strip the additive particles from the surface of the polymer particles. For example, the process of loading a sample of the mixture into a hopper and subsequently a compression molding machine may subject the coated particles to sufficient shock that the additive particles dissociate from the polymer particles and settle to the bottom of the hopper or compression molding machine. In the former, parts made from materials taken from the top of the hopper will have less or a lower concentration of the additive than those taken from the bottom. Similarly, in the latter, the bottom surface of the compression molded part will tend to have a higher concentration of the additive than the upper surface. In flow environments, for example in water treatment applications where polymer media is coated with various additives such as sequestering agents, antimicrobial agents, and the like, the water flow through the media, as well as shocks and vibrations upon the media vessel, may result in the dissociation of the additive and; consequently, the loss of the additive particles into the flow stream and the concurrent loss of efficacy/performance.
While dry mixing may be the simplest and least costly method of preparing such combinations, the most common is perhaps melt blending. Melt blending typically involves adding the additive particles and the polymer particles individually or as an intimate, dry mixture to an extruder whereby the additive is intimately incorporated into the polymer as it melts and proceeds through the extruder. Alternatively, the solid additive particles may be added to the polymer melt already in an extruder barrel through a second port further down the screw or barrel of the extruder. In both instances, the melt readily entombs the additive particles; however, oftentimes the additive, particularly when in powdered form, especially as a fine powder, is poorly dispersed in the polymer melt due to agglomeration thereof during the incorporation process and/or poor wetting. While relative particle size of the additive to the polymer material is not so much of a concern here since the additive is added to a melt of the polymer, the smaller the additive particle size, the greater the concern with poor wetting and dispersability. On the other hand, as the additive particle size increases, there is concern for wear and interference with the operation of the extruder and subsequent pelletizing equipment. Regardless, as a result of poor wetting and/or dispersability, the physical or performance properties associated with the presence of the additive may not manifest or may manifest in a non-uniform manner. Furthermore, such agglomeration may cause the appearance of physical imperfections in parts and articles made, directly or indirectly, from the melt, especially in the case of transparent or translucent materials and those used in forming films.
Typically, the foregoing concerns can be avoided or at least lessened by the preparation of concentrates or, as they are oftentimes referred to, masterbatches or pre-mixes wherein large amounts of the additive are combined, typically through melt blending, with the same or a different, more accommodating polymer and pelletized and those pellets subsequently sold for use in combination with, or directly combined with, virgin pellets of the ultimate polymer in a second extrusion process. In essence, the high additive concentration masterbatch is “let down” though a subsequent extrusion blending process. The product of the “let down” process is then pelletized for commercial sale of a final polymer composition or directly injected or extruded into various extrusion or molding equipment for manufacture of the end product(s)/article(s) or manufacture. Here, however, concerns arise with respect to compatibility of the masterbatch polymer with the ultimate polymer, if different, and, perhaps more importantly, with the sensitivity of the masterbatch polymer and/or additive to the repetitive high temperature extrusion cycles. The latter is especially of potential concern for organic and metalorganic additives, which, as opposed to most inorganic additives, tend to be much more heat sensitive. Such high temperatures may adversely affect the physical as well as performance characteristics of the additive or, in the case of heat activated additives, cause the premature activation thereof.
Solution blending overcomes many of these issues; yet introduces another set of issues altogether. Since most polymers are not water soluble, one must employ various organic solvents or co-solvents. Besides adding materials costs, the use of solvents, especially organic solvents, creates a number of additional concerns relative to environmental, health and safety precautions, most especially for capturing the solvent as it is evaporated to recover the polymer material. Furthermore, the additives themselves may be sensitive to or adversely affected by the solvents and recovery processes.
An additional pitfall of the aforementioned incorporation methods is the fact that certain additives are intended only for providing surface characteristics and/or are effective only if at the surface of the polymer particle or, in the case of articles of manufacture made thereof, the substrate surface of the so manufactured product. For example, glitter (small metal or metallic appearing flakes) on the surface of the polymer particle or article of manufacture will manifest its reflective/glittery appearance; however, that within the body or matrix of the polymer will not. Similarly, antimicrobial agents must either be present on the substrate surface or migrate through the polymer matrix to the substrate surface to be effective in providing antimicrobial performance. In the absence of the ability to migrate, that portion of the antimicrobial agent within the polymer matrix is unavailable to provide antimicrobial efficacy. This is especially of concern for inorganic antimicrobial agents, especially those that rely upon an ion-exchange type mechanism for performance. As a consequence, direct incorporation requires the use of much higher levels of the antimicrobial agent in order to achieve even a reasonable, though oftentimes short-lived, antimicrobial performance. Such higher loadings may adversely affect the physical properties of the polymer into which they are incorporated and add costs, a key consideration with, for example, ion-exchange type antimicrobial agents which tend to be fairly expensive. While dry blending to surface coat the polymer particles may overcome this concern, it introduces the problems previously mentioned above.
One method of addressing, at least in part, the aforementioned problems and pitfalls of direct incorporation and, where appropriate, dry blending is through the use of elevated temperatures in the dry blending process. Specifically, the dry blend is mixed at high speed and/or with direct elevation of temperature in the mixing vessel so as to elevate the mixture to a temperature at or above the melt temperature of the polymer or at least to that temperature at which the polymer becomes tacky. At this elevated temperature, the particles of polymer and additive become adhesively bonded to one another. This process may cause a deformation of the polymer particles, which may be detrimental for certain applications such as water treatment applications or powder coating applications. More significantly, this process causes an agglomeration and fusing of the polymer particles to themselves. Consequently, the product of this process must be screened in order to separate out the agglomerated particles which are then subjected to a further grinding operation to break apart the agglomerations. Depending upon the ultimate use of the treated particles, most often the reground particles must be re-entered into the coating process to ensure that those areas where another polymer particle had previously bonded are now coated with the additive materials. Besides the concerns with agglomeration, this coating process requires the use of specialized, especially coated equipment and strict/careful process controls, especially temperature controls, to ensure that the molten or tacky polymer particles do not adhere and bind to the mixing vessel, the mixing equipment or any other conduit or transport equipment before they are adequately cooled so as to no longer exhibit tacky or adhesive characteristics or, worse, to prevent the whole mass from fusing or congealing in the mixer vessel and the attendant consequences thereof.
Alternatively, the art has also suggested solution bonding and/or coating compositions as means to impregnate or coat the prepolymer or polymer particles with the additive: thereby, physically bonding the additive to the surface of the polymer particle. However, as mentioned above with respect to solution blending, these methods introduce new material(s) to the overall composition and/or employ material(s) that may raise environmental, health and safety issues as well as issues relative to the performance of the additives and/or the resultant polymer compositions themselves. Furthermore, these methods entail additional and oftentimes complex or costly processing steps and the attendant capital equipment needs as well as longer processing times. For example, solution impregnation is only commercially feasible for those solvents that are not overly toxic, expensive, difficult to handle, etc. and is limited to those prepolymer and polymer compositions that swell in the given solvent as well as those additives that are soluble in or easily suspended in the chosen solvent. Furthermore, one must employ appropriate means to contain and recover the solvents. Similarly, besides the introduction of the new materials and the costs associated therewith, coating processes require specialized coating apparatus and processes to ensure that the polymer particles are individually coated without, or without significant, agglomeration of the coated polymer particles. Consequently, neither is a truly viable, economical option.
Thus, there remains a need for a method of combining solid additives and polymer particles wherein the mixture remains stable, particularly in the case of marked particle size differences and irrespective of whether the combined materials are subject to vibration or shock or flow.
There also remains a need for a method of fixedly combining solid additives and polymer particles without directly incorporating the additive into the polymer.
Further, there remains a need for combining solid additives and polymer particles without the use of solvents and without the need for highly specialized and expensive equipment.
Finally, there remains a need for a method of fixedly combining solid additives and polymer particles which method is simple and does not result in any significant agglomeration and/or fusing of the polymer particles. In particular, there remains a need for a simple process for successfully binding an antimicrobial powder to the surface of prepolymer and polymer particles which avoids the use of additional chemical agents and additives, avoids concerns of agglomeration of the prepolymer and/or polymer particles, does not require the purchase and implementation of expensive/additional equipment, and can be accomplished at low cost.